A SIMPLE LIGHT-METER FOR MEASUREMENTS OF PAR (400 TO 700 NM) WITH FIBEROPTIC MICROPROBES - APPLICATION FOR P VS E-0(PAR) MEASUREMENTS IN A MICROBIAL MAT

A simple portable light meter for use with fiber-optic microprobes was developed. The meter has a flat spectral quantum responsivity for 400 to 700 nn light (photosynthetically available radiation, PAR). With s calar irradiance microprobes connected to the meter, it was possible t o directly measure photosynthetically available quantum scalar irradia nce, E-0(PAR), at <100 mu m spatial resolution and over a dynamic rang e from <1 to >1300 mu mol photons m(-2) s(-1). We used the new instrum ent for scalar irradiance measurements in microbial mats from a freshw ater lake (Lake Stigsholm, Denmark) and from a hypersaline pond (Eilat , Israel). Combined measurements of quantum scalar irradiance by fiber -optic microprobes and oxygenic photosynthesis by oxygen microelectrod es made it possible to measure gross photosynthesis as a function of t he prevailing scalar irradiance (Pvs Eo curves) at distinct depths wit hin an undisturbed hypersaline microbial mat of immotile unicellular c yanobacteria (Aphanothece spp.). Intense photosynthesis by the cyanoba cteria resulted in oxygen supersaturation and a 10-fold increase of ox ygen penetration in the illuminated mat (z(max) = 2.5 mm) as compared to the oxygen penetration in dark incubated mats (z(max) = 0.2 to 0.3 mm). The mat changed from a net oxygen consuming to a net oxygen produ cing community at an oxygen compensation irradiance of 14 to 26 mu mol photons m(-2) s(-1). The photic zone of the microbial mat was only 0. 6 mm deep due to a high attenuation of PAR. The diffuse vertical atten uation coefficient of E-0(PAR) was K-0(PAR) = 6.3 mm(-1). In the upper 0.2 mm of the microbial mat photosynthesis was photoinhibited at scal ar irradiance above 200 mu mol photons m(-2) s(-1). At 0.3 mm the stro ng light attenuation prevented inhibition in deeper layers of the micr obial mat and photosynthesis approached saturation at 35 mu mol photon s m(-2) s(-1). In the lower part of the photic zone, photosynthesis in creased linearly with E-0(PAR). Areal gross photosynthesis exhibited n o photoinhibition at high irradiance and started to approach saturatio n above a downwelling quantum irradiane of 97 mu mol photons m(-2) s(- 1).